1. Field of the Invention
The present invention relates to a rotary feed mechanism which can be advantageously used for a zoom lens barrel or the like.
2. Description of Related Art
In recent compact cameras having a zoom lens, a lens barrel, which is normally retracted in the camera body, is advanced to project from the camera body only when it is used. In order to reduce the overall length of the camera body, it is necessary to accommodate the lens barrel to thereby realize a thin and compact camera.
In such zoom compact cameras hitherto known, a movable lens barrel, which constitutes a part of a lens barrel, is screw-engaged by a stationary ring secured to the camera body through male and female multiple threads, so that the movable lens barrel can be moved with respect to the stationary ring. The movable lens barrel is provided on the outer peripheral surface thereof with a circumferentially extending engaging land which is provided thereon with the above-mentioned male multiple thread, inclined gears having a plurality of threads which are inclined in the same direction as the male multiple thread, and a single thread which constitutes a part of the male multiple thread being provided between the inclined gears. The camera is also provided with a series of gears having a plurality of pinions of small width which are secured to a common shaft and successively engaged by the inclined gears. When the series of gears are rotated, the movable lens barrel is rotated relative to the stationary ring to move in the forward or backward direction.
In conventional zoom compact cameras as mentioned above, the thread width of the inclined gears in the optical axis direction of the movable lens barrel is long enough to always engage with any one of the small width pinions, regardless of the axial position of the movable lens barrel. However, in order to increase the length of projection of the movable lens barrel from the camera body, it is necessary to reduce the thread width of the inclined gears in the optical axis direction. If the width of the inclined gears is minimized, the width of engagement between the small width pinions and the inclined gears is also reduced. In the worst case scenario, no inclined gear can engage with the small width pinions.